The multiplication of animal cells in vitro requires the presence of macromolecular growth factors in the culture medium. Serum, the usual source of these factors for cell growth, is extremely complex, and the identification and characterization of serum's growth-promoting components has been very difficult. In these proposed studies, a purified growth factor, called multiplicationstimulating activity (MSA) will be used to investigate various parameters of cell growth regulation. MSA is produced by the BRL-3A line of rat liver cells which grow in the complete absence of serum. Conditioned medium from these cells is used as the source for the purification of MSA, which is a protein of about 9,000 molecular weight. MSA will stimulate DNA synthesis and growth of a variety of cell types, has weak insulin-like activity and is a representative of the somatomedin family of growth regulatory hormones. Various parameters of growth regulation will be examined with MSA, including biological responses as well as labeled MSA binding and utilization. In addition, we have been successful in purifying a somatomedin, or MSA carrier protein, from conditioned medium by using affinity chromatography procedures with immobilized MSA linked to Sepharose 4B. We plan to characterize this protein and compare it with somatomedin carrier complexes isolated from serum and examine its role in modulating the biological activity of MSA in vitro. Using similar affinity chromatographic procedures, it will also be possible to isolate the MSA receptor from cell surfaces and characterize its properties.